Two-probe systems are generally already known. Thus, U.S. Pat. No. 3,969,932 discloses a method as well as an arrangement for monitoring the activity of a catalytic converter in the exhaust gas system of an internal combustion engine. Here, the probe forward of the catalytic converter operates for the actual closed-loop control; whereas, the probe rearward of the catalytic converter operates in combination with the probe forward of the catalytic converter to monitor this converter. For this purpose, changes in the output signals of both lambda probes are detected and the time delay between the two changes is evaluated as a measure of the catalytic activity.
U.S. Pat. No. 4,007,589 shows a similar arrangement. Here too, a conclusion is drawn as to the effectiveness of the catalytic converter by means of a comparison of the two probe signals.
U.S. Pat. No. 4,739,614 also discloses a two-probe system for an internal combustion engine. Here, the probe disposed upstream is used for the closed-loop control of the actual air/fuel ratio In addition, an open-loop control variable is formed in dependence upon the output signal of the probe rearward of the catalytic converter. At the end of the abstract of this patent, it is stated that the computation of the air/fuel ration is prevented when the probe disposed downstream is in an abnormal condition.